The present invention relates to a connecting structure for connecting a ceramic shaft to a metallic shaft. More particularly, it relates to a connecting structure for connecting a ceramic shaft to a metallic shaft, which is suitable for driving the ceramic shaft attached with a ceramic butterfly valve body of a butterfly valve used at a high temperature.
A compressed air having a high temperature such as 1200.degree. C. and a high pressure such as 3.5 kg/cm.sup.2 (guage pressure) is blown into tuyeres of a blast furnace for manufacturing iron at a high velocity such as 120 m/s and the blast furnace is provided with about twenty tuyeres. In such a blast furnace, it is desirable to separately control a flow rate of air of high temperature, pressure and velocity for each tuyere with use of valves.
To satisfy the above-described purpose, an attempt to use a valve having a metallic valve body was made. However, it could not be in practical use because the metallic valve body is poor in heat resistance. Another attempt of cooling a metallic valve body with water failed to put it to practical use due to large heat loss in the valve body. On the other hand, it can be considered that ceramics having excellent heat resistance are used to form a valve body and a driving shaft in one piece. It is, however, impossible to manufacture such a complicated product in shape with ceramics at the present technique. In view of difficulty in manufacturing a desirable valve, there are proposals that a valve body including a shaft is made of ceramics; a driving shaft is made of metal and both the shafts are connected together. As one of the proposals, there is known a method of connecting a ceramic shaft to a metallic shaft with an adhesive. This method has, however, disadvantages that an expensive and special adhesive is required and a pre-treatment step used is complicated. There is another method of subjecting a metallic shaft to shrinkage-fitting onto a ceramic shaft. However, the method requires a highly accurate and delicate technique to adjust the quantity of shrinkage and the method sometimes causes an extensive change in connecting strength due to variation in temperature. There is also known another method such that a recess (or a projection) for a pin or a key in a ceramic shaft is formed to engage it with an annular part having the corresponding shape of a metallic shaft. With this method, the ceramic shaft has to be finished into a complicated shape and in addition, concentration of stress is produced at the recess (or the projection) of the ceramic shaft when used thereby impairing reliability. Further, a gap is produced at the connecting part due to change in temperature to result in an excessive stress.
In the construction for connecting a ceramic shaft to a metallic shaft as above-mentioned, a rotational torque from the metallic shaft must be certainly and powerfully transmitted to the ceramic shaft. Since the valve body is exposed to an air flow having a high velocity, there easily takes place vibration. The valve should, nevertheless, have the connecting structure without causing looseness and with a high rigidity. Further, it is desirable that the connecting structure is easily assembled, disassembled or can permit replacement of parts.
There has not been practically used a control valve for a high temperature, high pressure fluid, since a connecting structure satisfying the problems as abovementioned has not be proposed.